1. Field of the Invention
The present invention relates to a liquid crystal display device provided with a TFT (thin film transistor) or the like as a switching element, and to a method for manufacturing the same.
2. Description of the Related Art
At present, a liquid crystal display device generally has a structure that two glass substrates having a transparent electrode face to each other and liquid crystal is sandwiched between the two substrates. In order to keep a distance between the two substrates constant, spacers of uniform particle diameter such as plastic beads are scattered between the substrates.
This structure has such a general problem that, when orientation of the liquid crystal molecules around the spacers scattered between the two substrates is disturbed, light leaks from peripheries of the spacers where the orientation is disturbed, and accordingly the contrast tends to be degraded.
Further, it is difficult to uniformly scatter the spacers between the substrates. In addition, irregularities due to lines, switching elements, color filters and so on between the substrates make the sealed-in liquid crystal layer nonuniform in thickness, depending on arrangement positions of the spacers, which causes display degradation and reduction in yield of product.
As measure for such problems, it is proposed to form columnar spacers by using a photoresist or the like.
However, in the case where a resin for an orientation film is applied after formation of the columnar spacers, and rubbing treatment is conducted in which the resin film for an orientation film is rubbed with a rubbing cloth to form a lot of fine grooves on the resin film for an orientation film along one direction, there arises a problem that part of the resin film is not rubbed due to protrusions of the spacers and the part becomes an orientation-disturbed region, which degrades a display quality. In addition, the columnar spacers are dropped from the substrates due to the rubbing treatment.
To solve these problems, the following methods are disclosed in Japanese Unexamined Patent Publications JP-A 9-73088 (1997) and JP-A 6-175133 (1994).
As to the orientation disturbance traced in the protrusions of the spacers as mentioned above, such a method is disclosed in both the publications that spacers are placed at positions where orientation-disturbed regions starting at spacers do not extend to pixels so that an orientation failure occurs only in portions which have no direct bearing on the display quality, using lines and light shielding regions on the substrates.
In addition, JP-A 9-73088 discloses a method that in order to reduce the resistance sustained by pile of the rubbing cloth and decrease the orientation failure, the spacers are made to have a vertex at a part coming into contact with the rubbing cloth, a smooth taper shape, and a cross-sectional shape of an ellipse, triangle, rhombus or the like.
Furthermore, JP-A 9-73088 discloses a method that in order to solve the problem that the spacers are dropped from the substrates, the shape of the spacers is made to be an elliptic cylinder and the longitudinal direction of the elliptic cylinder is made to be parallel to a rubbing direction, whereby a mechanical strength of the spacers is maintained while a load on the spacers is minimized.
However, since adhesion strength of the spacers to the substrate is not taken into consideration in any method disclosed in the two publications, the spacers may be dropped due to a pressure of the pile in the rubbing treatment.
Particularly, the method of JP-A 9-73088 has a problem that the spacers are disposed on a light-shielding layer made of a black resin, so that the adhesion strength of the spacers to the light-shielding layer is extremely low and hence the spacers are easily dropped from substrates in the rubbing treatment. This is because an upper surface of the light-shielding layer made of a black resin is usually flat and hence the adhesion strength of the spacers thereto is low. Further, although it is designed that the spacers have a smooth shape so as to minimize a load from the rubbing cloth to the spacers, the adhesion strength of the spacers to the substrate face is not enhanced, and therefore the problems are not essentially solved. Furthermore, in order to make the spacers have a sectional shape of an ellipse, triangle, rhombus or the like, precise and complicated operations of photolithography, etching process and so on are required, so that there are problems of increase in cost and reduction in yield.
On the other hand, in JP-A 6-175133 the spacers are formed on uneven regions which are between the pixels or on the switching element to obtain a wide adhesion area for the purpose of enhanced adhesion strength, however, the spacers are formed on the orientation film with the result that a remarkable improvement of the adhesion strength cannot be achieved. Moreover, since the rubbing treatment is conducted after the formation of spacers, there remains the problem that the spacers are dropped from the substrates due to the pressure of the rubbing cloth. Further, since the surface of the orientation film is exposed to a chemical for forming spacers in a spacer forming process, such a serious problem arises that a force for regulating the orientation of liquid crystal molecules is reduced, the display quality is degraded due to a printed after-image and so on. Additionally since the art of JP-A 6-175133 takes no measure in connection with the shape of the spacers, an orientation failure which accompanies a rubbing failure cannot be avoided.